Thermoelectric devices are available in a variety of materials and configurations. Referring to FIG. 1, a conventional thermoelectric device is shown generally as reference numeral 10. Thermoelectric device 10 includes a plurality of thermoelectric couples 12a and 12b (only two of which are shown in FIG. 1). As shown, each thermoelectric couple comprises an n-type semiconductor element 14 and a p-type semiconductor element 16. The n-type and p-type semiconductor elements 14, 16 are electrically connected in series via connection plates 18a, 18b, 18c, 18d and 18e, as shown in FIG. 1. In addition, the n-type and p-type semiconductor elements 14, 16 are thermally connected in parallel between a top plate 20 and a bottom plate 22. Within each thermoelectric couple, the n-type and p-type semiconductor elements 14, 16 are spaced a short distance apart (represented by distance d in FIG. 1) in order to minimize the overall size of the device. For example, a typical distance between n-type and p-type semiconductor elements 14, 16 is in the range of 0.5 mm to 2.0 mm.
In general, thermoelectric devices are used in two different modes of operation: an electrical power generation mode and a heating/cooling mode. In this regard, thermoelectric device 10 includes contact points 24 and 26 that provide connections to either a load (for the electrical power generation mode) or a power source (for the heating/cooling mode), wherein the load or power source are shown generally as reference numeral 40. In the electrical power generation mode, heat (e.g., heat from the sun) is applied to top plate 20 whereby electrical power is delivered to the load connected to contact points 24 and 26. In the heating/cooling mode, the power source is connected to contact points 24 and 26 whereby the temperature of each of top and bottom plates 20, 22 changes to provide the desired heating or cooling effect.
While conventional thermoelectric devices offer advantages that are not provided by other technologies, they are still relatively inefficient. In the electrical power generation mode, the efficiency of a thermoelectric device increases with a greater temperature difference between top plate 20 and bottom plate 22. This is accomplished by selecting the materials of each of the n-type and p-type semiconductor elements 14, 16 to have a low thermal conductivity (e.g., a thermal conductivity of 0.026 W/cm° K or lower). However, materials with a low thermal conductivity generally have a low Seebeck coefficient (e.g., a Seebeck coefficient of 140 μV/° K to 250 μV/° K for the p-type semiconductor element and a Seebeck coefficient of −75 μV/° K to −200 μV/° K for the n-type semiconductor element). For example, Bi2Te3 is commonly used for the p-type semiconductor element 16, which has a thermal conductivity of about 0.020 W/cm° K and a Seebeck coefficient of about 240 μV/° K. Also, Bi2Te3+0.1% I is commonly used for the n-type semiconductor element 14, which has a thermal conductivity of about 0.0256 W/cm° K and a Seebeck coefficient of about −184 μV/° K. In addition, materials with a low thermal conductivity generally have a low electrical conductivity, which does not provide optimum efficiency. While efforts have been made to develop materials with a low thermal conductivity and higher Seebeck coefficients and electrical conductivity, there are still limits on the efficiency that can be achieved with the use of these materials. In particular, the efficiency of a conventional thermoelectric device is typically less than about 15%. There are similar limits on the coefficient of performance for cooling or refrigeration (COPR) and the coefficient of performance for heating (COPH) that may be achieved with the use of such materials when a conventional thermoelectric device is operated in the heating/cooling mode.
Another measure of the performance of a thermoelectric device when operating in the electrical power generation mode is ZT, where Z is the figure of merit of the device and T is the mean temperature of the device. The ZT value of a conventional thermoelectric device is typically less than about 1.5 and most typically about 1, although a recent publication claims to have attained a ZT value of about 2.2 at 600° K. See High-performance bulk thermoelectrics with all-scale hierarchial architectures, Nature 489, 414-418 (Sep. 20, 2012). Even if such claims are accurate, a ZT value of 2.2 is still well below desired levels.
U.S. Pat. No. 5,006,178 to Bijvoets discloses an alternative thermoelectric device that includes a plurality of thermoelectric elements electrically connected in series and thermally connected in parallel. Each thermoelectric element is provided with two element halves of opposite conductivity types. Each element half has two semiconducting end pieces and an electrically conducting intermediate piece. Bijvoets teaches that the semiconducting end pieces are made of typical semiconductor materials, such as BiTe. Again, there are limits on the efficiency, ZT value, COPH and COPR that can be achieved with these materials.